1. Field of the Invention
The present invention relates to flexible fluoropolymer tubes which exhibit excellent properties such as strength, wear resistance and dimensional stability when subjected to repeated flexing due to vibration, bending, or the like.
2. Description of Related Art
Polytetrafluoroethylene (PTFE) has demonstrated utility in many areas. As an industrial material, such as for gasketing or tubing material, PTFE has exhibited excellent utility in harsh chemical environments which normally degrade many conventional metals and polymeric materials. The non-reactive nature of PTFE maintains purity during use in articles fabricated therefrom, because such PTFE articles typically containing no plasticizers, fillers, stabilizers or anti-oxidants that could leach out and react with process fluids, powders, or manufactured components from high value industries including semiconductor fabrication processes and pharmaceutical production. PTFE is also usable over a broad temperature range, from as high as about 280.degree. C., or higher, to as low as about -273.degree. C.
Tubes of PTFE have been fabricated by a variety of methods. Typically, PTFE tubes are manufactured via paste extrusion in the presence of an organic lubricant, followed by lubricant removal, amorphously locking at temperatures above the crystalline melt point of PTFE, and further processing. U.S. Pat. No. 4,267,863 to Burelle and U.S. Pat. No. 3,225,129 to Taylor et al. discuss the fabrication of rigid, non-porous PTFE tubes by winding thin, calendered PTFE tapes onto a metal mandrel, followed by heating the layered tape tube to a temperature above the crystalline melt of PTFE for a time sufficient to achieve intra-layer adhesion. Moreover, large diameter tubes can be molded directly or machined from molded rods produced from granular PTFE resins. PTFE tubes fabricated using the aforementioned techniques have shown utility as liners for expansion joints, as described in, for example, U.S. Pat. No. 4,536,018 to Patarcity. Such conventional non-porous PTFE tubes fabricated through extrusion, tape winding, or compression molding exhibit poor mechanical properties, such as poor flexibility, low tensile strength, and low flexural strength. Accordingly, despite a number of highly desirable performance characteristics, the use of conventional non-porous PTFE tubes is generally limited to applications requiring only limited flexibility.
Flexural stresses applied to tubular components, particularly those stresses experienced under repeated bending or rapid axial compression and recovery resulting from cyclic movement or vibration of apparatus onto which the tube is coupled, are a particular problem for conventional non-porous PTFE is and PTFE composite tubes. Specifically, these materials typically weaken as a result of flexural stress and/or abrasion associated with the continuous axially oriented flexing or folding, which leads to the development of cracks and flaws in the tube and eventually results in catastrophic failure of the tube.
Polytetrafluoroethylene may be produced in a porous, expanded form as taught in U.S. Pat. Nos. 3,953,566, 3,962,153, and 4,187,390 to Gore. The membranes and tubes described therein have a microstructure comprised of nodes interconnected by fibrils. The formation of tubes is carried out by extruding a mixture of PTFE and liquid lubricant, removing lubricant from the resulting tubular extrudate and expanding the extrudate by stretching at a suitable rate at a temperature between about 100.degree. C. and 325.degree. C. The resulting tube may preferably be subjected to amorphous locking while the tube is longitudinally restrained. This processing creates desirable orientation of the material and, correspondingly, strength, primarily in the longitudinal direction. However, for applications requiring hoop strength or burst strength, such as those experiencing high internal pressure, these tubes often may not include sufficient circumferential wall strength to meet desired performance needs.
Tubes formed from sheets of stacked layers of expanded PTFE have been formed by conventional tube seaming sealing techniques, such as end-to-end butting and sealing, skiving of ends to seal, and overlapping of ends to form a seam, such as shown in FIGS. 1A, 1B and 1C, respectively. The ends are joined by any conventional sealing technique, such as by the use of an adhesive, by densifying and melting the ends together, or the like. One example of tubes formed by skiving the ends of a flat sheet and bonding the skived ends with an adhesive to form a seamed tube are the tubes commercially available from Helms Industrial Supply, Inc., and fabricated from flat sheets of stacked expanded PTFE layers sold by W. L. Gore & Associates, Inc. (Elkton, Md.), as GR SHEET.RTM. gasketing material.
These tubes have been incorporated as pipe connectors in industrial systems, whereby the seamed tubes are placed between two pipes and affixed, such as by clamping, on the ends of the pipes. Two exemplary configurations of this set-up showing hose clamps 50 and a pipe coupling 52 having bolts 53, to clamp onto and hold the tube 54 in place, respectively, are shown in FIGS. 2A and 2B.
Membranes of porous expanded PTFE having uniaxially, biaxially, or multi-axially oriented fibrils as described in the aforementioned U.S. Patents to Gore have also been used in the fabrication of porous tubes by winding expanded PTFE membranes onto a mandrel at a temperature above the crystalline melt temperature of the PTFE for a period of time to achieve adhesion of the layers. Such wrapping may be carried out as, for example, an external helical wrap onto the expanded PTFE tubing described above to increase the hoop strength, such as is commercially available from W. L. Gore & Associates, Inc. (Flagstaff, Ariz.), as vascular grafts sold under the trade name GORE-TEX.RTM., or as a wrapped layer between an inner and outer expanded tube as disclosed in, for example, U.S. Pat. No. 4,787,921 to Shibata.
Expanded PTFE wound tubing and composite tubing produced in the aforementioned ways have been deemed advantageous for possessing strength in both the longitudinal and circumferential directions, diametrical flexibility, thin wall thicknesses of up to 0.25 mm and small diameters of up to 25.4 mm, and for achieving collapsibility. These tubes have been found to be suitable in such diverse areas as intraluminal vascular grafts, filter elements, or catheters as described in PCT Publication No. WO 95/0555, gastroscope introducers as described in EPO Publication No. 0 605 243, and for gas permeation in applications such as degassing tubes as described in U.S. Pat. No. 4,787,921 to Shibata.
Although the prior art tubular articles work well in the applications for which they were intended, the prior art fails to teach the improved tubes of the present invention which provide novel utility in a variety of industrial applications requiring high flex life, high strength, high thermal and chemical resistance, and high purity.
Accordingly, it is a primary purpose of the present invention to provide novel tubes comprising expanded PTFE which exhibit enhanced flex life compared to conventional flexible tubing materials.
It is a further purpose of the present invention to provide novel tubes comprising expanded PTFE which exhibit enhanced flex life while having wall thicknesses and diameters which were heretofore unachievable based upon the teachings of the prior art.
It is a further purpose of the present invention to provide novel tubes comprising expanded PTFE which incorporate surface textures, such as corrugations and the like, to provide enhanced performance in a wide variety of applications.
It is a further purpose of the present invention to provide novel production techniques for fabricating the novel tubes of the present invention.
These and other purposes of the present invention will be apparent based on the following description.